When a strip is being hot rolled for instance from a starting thickness of 30 mm to 40 mm to a finished thickness of 2 mm to 10 mm prior to cold finish-rolling, it is necessary to make constant corrections in the strip shape. The rotation speed of the rolls, the roll spacing, the tension in the strip, and so on can be adjusted according to well established procedures (see for instance copending application Ser. Nos. 352,520 filed Feb. 26, 1983, 379,890 filed May 19, 1985, 558,165 filed Dec. 5, 1983, and 587,231 filed Mar. 07, 1984) to control three major aspects of the workpiece: its thickness, its planarity, and its width. The thickness is fairly critical as is the planarity, and the width is normally easy to control.
Thickness corrections in long workpieces such as strips or long plates are normally also reflected in changes in workpiece width. The extent of deformation in the width direction, that is parallel to the plane of the workpiece and perpendicular to its longitudinal travel direction through a stand, is a function of the geometry of the roll stand, the size of the rolls, the workpiece thickness, temperature, and composition, as well as the friction between the workpiece and the rolls and the tension in the workpiece.
Corrections of planarity are typically associated with longitudinal stretching or elongation of the strip in the travel direction. Such a correction does not substantially affect workpiece width but has some effect on workpiece thickness, uniformly reducing it.
As mentioned the steps taken to control thickness also affect planarity. In the cold-rolling operation following the hot-rolling one it is necessary to start with a high-tolerance workpiece both with regard to thickness uniformity and to planarity to produce a high-tolerance end product. Any attempt to correct thickness irregularities during cold rolling is reflected in wide deviations from planarity. Even a correction of a thickness nonuniformity amounting to one hundredth of the workpiece thickness can result in a bulge in the workpiece some 22 mm high and 1 m long, albeit the workpiece is of uniform thickness. Thus extreme care must be taken to supply a relatively planar workpiece of substantially uniform thickness to the cold-rolling operation.
When the thickness is adjusted in the first stand of a hot-rolling line normally comprising five or more stands, the correction is normally not perceptible at the downstream output end of the line, that is the downstream stands eliminate the correction. The planarity does not change however. Trying to make any substantial changes in the nip shape of the output stand leads to simultaneous changes in thickness and planarity. Thus it has been assumed that the thickness can only be permanently changed in the output stand, and that this must be done gradually with the runs of the workpiece through the equipment. Hence producing a hot-rolled workpiece satisfying high tolerances regarding thickness uniformity and planarity or levelness has a been a difficult and expensive process, albeit such a hot-rolled workpiece is the essential starting material needed for the production of high-tolerance cold-rolled strip.